The present invention concerns a pneumatic dosage or metering device for metering powdered materials, for example in connection with feeding aluninum oxide and aluninum fluoride to an electrolysis cell for the production of aluninum, a container in the form of a relatively long, closed channel or similar structure with a fluidizing element fitted in the base of the channel for air supply for fluidization of material located in the channel. In addition, the container is fitted with an inlet for feeding material from a store in the form of a day tank, silo or similar source and an outlet for metered discharge of material from the channel.
There are previous devices of the above type in which transportation and metering take place in accordance with the fluidization principle. The prerequisite for being able to use such devices is that the material which is to be transported and metered is fluidizable, i.e. that it is pulverulent and has such granulometry and cohesion that the inlet air flow speed slowly forces decohesion between particles and a reduction of internal friction forces, causing the thus formed suspension to behave essentially as a homogenous fluid. Such materials include aluninum oxide used for smelting electrolysis as mentioned above, cements and plaster, slaked or unslaked lime, calcium fluoride, charger for plastic and rubber, catalysts, powdered carbon, sodium hydrogen-sulphate, phosphates, poly-phosphates, pyrophosphates, metal powder, pulverulent plastic materials, food products such as flour, milk powder, sugar, etc.
The inventors have been working on the transportation and metering of fluidizable material for a number of years during which they have tried to find the simplest, most reasonably priced and most durable solutions in practice. In this connection, attention is directed to, Norwegian Patent No. 162774 corresponding to U.S. Pat. No. 4,930,691 and Norwegian Patent Application No. 911178, corresponding to U.S. Pat. No. 5,360,297. The first of these arrangements relates to a metering device based on a type of volumetric metering in which, by means of fluidization, material flows from a store via a fluidizing channel to a fluidizing container where it is collected in a certain quantity, depending on the size of the container. When the container is full, the air supply to the fluidizing channel is stopped and air is supplied to the container so that the material in the container is fluidized and consequently discharged. This is, in fact, a simple solution which provides accurate doses but the size of the doses is difficult to adjust.
The second arrangement relates to a fluidizing channel for automatic level control for which an inactive zone, i.e. a zone which is not fluidized, is used in the area below the channel inlet for the fluidizable material. This channel cannot be used for metering purposes in the form in which it is shown and described.